1. Field of the Invention
The present invention relates to a radar system for observing a moving object and the atmospheric air and measuring a velocity thereof, and to a coherent integrating method thereof.
2. Description of the Related Art
This type of conventional radar system transmits signals which are pulse-modulated for ranging, wherein electromagnetic waves such as light waves and radio waves serve as carrier waves. Then, the electromagnetic waves reflected by an observation target are received at an interval of a delay time corresponding to a distance from the measurement target. The receiving signal having a time equal to a pulse width is subjected to fast Fourier transform (FFT), thereby integrating the signal per Doppler frequency and measuring a moving velocity of the observation target. Therefore, an integration time for improving an S/N ratio is restricted by a transmission pulse width. Even when expanding the pulse width, the velocity of the observation target gradually changes with an elapse of time, and hence it is difficult to obtain sufficient effect in improving the S/N ratio by the coherent integration.
This type of conventional coherent radar system will be explained with reference to the drawings. FIG. 6 is a block diagram showing a construction of the conventional radar system disclosed in, e.g., U.S. Pat. No. 5,237,331.
Referring to FIG. 6, the radar system includes a transmitter/receiver 1, an A/D (Analog to Digital) converter 2, a range gate 3 for extracting a receiving signal with a delay time corresponding to a distance from an observation target, a Doppler FFT unit 4 for integrating the signal per Doppler frequency by executing a fast Fourier transform (FFT) of the receiving signal within a pulse width observed within the same range, and measuring a moving velocity of the observation target, and a PDI (Post Detection Integration) unit 5 for executing an incoherent integration (PDI) of a result of the FFT processing which has been gained by a series of processes described above from the receiving signals obtained per transmission pulse.
Next, an operation of the conventional radar system will be described with reference to the drawings. FIG. 7 is a timing chart showing a transmitting/receiving operation of the radar system illustrated in FIG. 6.
As shown in FIG. 7, the radar system repeatedly transmits electromagnetic waves which are pulse-modulated with a pulse width xcfx84 at a pulse interval xcfx84PRI (as in the case of the prior art system). The transmitter/receiver 1 detects a phase of the receiving signal, and the A/D converter 2 converts the receiving signal into a digital signal at a sampling interval xcfx84S. Then, the range gate 3 extracts the receiving signal having a time corresponding to the pulse width xcfx84 at an interval of a delay time xcfx84d corresponding to a distance from the observation target. This operation is repeated the same number of times as the number of transmission pulses. At this time, the digitized receiving signal can be expressed in the following formula (1) by use of a pulse number n and a sample number m in the pulse width.
Sxcfx84d(n,m)xe2x80x83xe2x80x83[Formula 1]
m=0,1,2, . . . , 2Mxe2x88x921
n=0,1,2, . . . , Nxe2x88x921
This receiving signal Sxcfx84d (n, m) is converted into a Doppler spectrum Sxcfx84d (n, l) by the Doppler FFT unit 4. The spectrum Sxcfx84d (n, l) is given by the formula (2), where l=0, 1, 2, . . . , 2Mxe2x88x921 indicates a filter number of FFT, i.e., a Doppler frequency component number. According to the fast Fourier transform (FFT), the number of samples on the time-axis is 2Mxe2x88x921 equal to the number of samples on the frequency-axis.                                                         S                              τ                ⁢                                  xe2x80x83                                ⁢                d                                      ⁡                          (                              n                ,                l                            )                                =                                    ∑                              m                =                0                                                              2                  ⁢                  M                                -                1                                      ⁢                          xe2x80x83                        ⁢                                                            S                                      τ                    ⁢                                          xe2x80x83                                        ⁢                    d                                                  ⁡                                  (                                      n                    ,                    m                                    )                                            ⁢                              ⅇ                                                      -                    j2π                                    ⁢                                      ml                                          2                      ⁢                      M                                                                                                          ⁢                  
                ⁢                              l            =            0                    ,          1          ,          2          ,          …          ⁢                      xe2x80x83                    ,                                    2              ⁢              M                        -            1                          ⁢                  
                ⁢                              n            =            0                    ,          1          ,          2          ,          …          ,                      N            -            1                                              [                  Formula          ⁢                      xe2x80x83                    ⁢          2                ]            
The Doppler spectrum Sxcfx84d (n, l) obtained per pulse hit is converted into electric power by a square-law detection as shown in the formula (3), and is PDIed, i.e., receives an addition of the number of pulse hits for the every same Doppler frequency component.                                                         S              out                        ⁡                          (              l              )                                =                                    ∑                              n                -                0                                            N                -                1                                      ⁢                          xe2x80x83                        ⁢                                          "LeftBracketingBar"                                                      S                                          τ                      ⁢                                              xe2x80x83                                            ⁢                      d                                                        ⁡                                      (                                          n                      ,                      l                                        )                                                  "RightBracketingBar"                            2                                      ⁢                  
                ⁢                              l            =            0                    ,          1          ,          2          ,          …          ⁢                      xe2x80x83                    ,                                    2              ⁢              M                        -            1                                              [                  Formula          ⁢                      xe2x80x83                    ⁢          3                ]            
Generally, a greater effect in improving the S/N ratio which is gained by coherently adding the number of pulse hits per frequency component 1 of the obtained Doppler spectrum Sxcfx84d (n, l) in phase as a complex number is than by the PDI process of adding the number of pulses after the square-law detection. As compared with the pulse width, however, a pulse interval shows a several 10-fold through 100-fold breadth, and hence the coherent integration is difficult to attain due to a disturbance in terms of phase which might be caused by fluctuations in velocity of the target in that span. Therefore, in this type of conventional radar system, as explained above, there are conducted the intra-pulse-width coherent integration and the between-pulses incoherent integration (PDI).
As discussed above, this type of conventional radar system does not has a mechanism or a unit for compensating a change in the Doppler frequency due to variations in the moving velocity of the target when coherently integrating the receiving signal based on the FFT, with the result that the integration time is restricted short and the sufficient effect in improving the S/N ratio can not be obtained.
The problem inherent in the conventional radar system described above is that the coherent integration time is restricted due to the fluctuations in velocity of the observation target object or atmospheric air, and, even if the coherent integration is performed over a long period of time, the sufficient effect in ameliorating the S/N ratio is unable to be obtained.
It is a primary object of the present invention, which was devised to obviate the problems described above, to provide a radar system capable of sufficiently improving an S/N ratio with respect to an observation target that is difficult to execute a coherent integration between a plurality of pulses with a small influence by fluctuations in velocity of an observation target because of the fluctuations being short in time within a pulse width but with a disturbance in terms of phase of a receiving signal which might be caused by fluctuations in velocity thereof between the pulses.
To accomplish the above object, according to a first aspect of the present invention, a radar system comprises a transmitter/receiver for transmitting electromagnetic waves which are pulse-modulated with a predetermined pulse width at a predetermined pulse interval, and detecting a phase of a receiving signal reflected from an observation target, an A/D converter for converting the receiving signal into a digital signal at a predetermined sampling interval, a range gate for extracting the receiving signal having a time corresponding to the predetermined pulse width at an interval of a delay time corresponding to a distance from the observation target, a data dividing unit for dividing the receiving signals into two groups of data, which come from the observation target located at the same distance, a first Doppler FFT unit for fast-Fourier-transforming one group of data of the two groups of divided data, a second Doppler FFT unit for fast-Fourier-transforming the other group of data of the two groups of divided data, a complex conjugating unit for taking a complex conjugate of an output of the second Doppler FFT unit, a complex multiplying unit for performing a complex multiplication for the every same Doppler frequency component with respect to an output of the first Doppler FFT unit and an output of the complex conjugating unit; and a complex adder for repeating the observation and processing a plural number of times, and coherently adding an obtained result of the complex multiplication the same number of times as the repetitions with respect to the same Doppler frequency component.
According to a second aspect of the present invention, in the radar system according to the first aspect of the invention, the data dividing unit may include a switch for dividing the receiving signals outputted from the range gate into two groups of even-numbered data and odd-numbered data, a first memory for storing the even-numbered or odd-numbered data, and a second memory for storing the odd-numbered or even-numbered data. The first Doppler FFT unit fast-Fourier-transforms the data stored in the first memory, and the second Doppler FFT unit fast-Fourier-transforms the data stored in the second memory.
According to a third aspect of the present invention, in the radar system according to the first aspect of the invention, the data dividing unit may be a switch for dividing the signals outputted from the range gate into two groups of data based on a former half of an observation time and a latter half of the observation time. The radar system may further comprise a delay unit, provided posterior to the first Doppler FFT unit, for taking synchronism of an output of the first Doppler FFT unit for fast-Fourier-transforming the former half of the data and an output of the second Doppler FFT unit for fast-Fourier-transforming the latter half of the data.
According to a fourth aspect of the present invention, a radar system comprises a transmitter/receiver for transmitting repeatedly electromagnetic waves which are pulse-modulated with a predetermined pulse width at a predetermined pulse interval, and detecting a phase of a receiving signal reflected from an observation target, an A/D converter for converting the receiving signal into a digital signal at a predetermined sampling interval, a range gate for extracting the receiving signal having a time corresponding to the predetermined pulse width at an interval of a delay time corresponding to a distance from the observation target, a Doppler FFT unit for fast-Fourier-transforming two groups of data in time-division based on a former half of an observation time and a latter half of the observation time with respect to the signals outputted from the range gate, a delay unit for synchronizing an output of a result of the FFT processing of the former half of the data with a result of the FFT processing of the latter half of the data, a complex conjugating unit for taking a complex conjugate of an output of the Doppler FFT unit, a complex multiplying unit for performing a complex multiplication for the every same Doppler frequency component with respect to an output of the delay unit and an output of the complex conjugating unit, and a complex adder for repeating the observation and processing a plural number of times, and coherently adding an obtained result of the complex multiplication the same number of times as the repetitions with respect to the same Doppler frequency component.
According to a fifth aspect of the present invention, a radar system comprises a transmitter/receiver for transmitting repeatedly electromagnetic waves which are pulse-modulated with a predetermined pulse width at a predetermined pulse interval, and detecting a phase of a receiving signal reflected from an observation target and A/D converter for converting the receiving signal into a digital signal at a predetermined sampling interval, a range gate for extracting the receiving signal having a time corresponding to the predetermined pulse width at an interval of a delay time corresponding to a distance from the observation target, an initial phase compensating unit for compensating a phase so that an initial phase of the receiving signal coming from the observation target located at the same distance becomes zero, a Doppler FFT unit for fast-Fourier-transforming the receiving signal after compensating the phase, and a complex adder for repeating the observation and processing a plural number of times, and coherently adding an obtained result of the FFT processing the same number of times as the repetitions with respect to the same Doppler frequency component.
According to a sixth aspect of the present invention, a coherent integrating method of a radar system, comprises a step of extracting a receiving signal sampled within one pulse per transmission pulse from receiving signal data, a step of dividing the extracted receiving signal data for one pulse hit into two groups of data, a step of fast-Fourier-transforming each piece of data among the two groups of the divided receiving signal data, a step of performing a complex conjugating operation of one of two FFT processing results, a step of executing a complex multiplication of the other FFT processing result of the two FFT processing results and a result of the complex conjugating operation, and a step of repeatedly executing the processes up to the complex multiplication with respect to an arbitrary number of transmission pulses, and performing a complex addition of an obtained result of the complex multiplication for the every same Doppler frequency component.
According to a seventh aspect of the present invention, in the coherent integrating method according to the sixth aspect of the invention, the dividing step may be to divide every set of the extracted receiving signal data for one pulse hit into even-numbered data and odd-numbered data.
According to an eighth aspect of the present invention, in the coherent integrating method according to the sixth aspect of the invention, the dividing step may be to divide the extracted receiving signal data for one pulse hit into two groups of data based on a former half of a pulse width and a latter half of the pulse width, and the method may further comprise a step of delaying the former half of the fast-Fourier-transformed data by a predetermined time in order to take synchronism of the former half of the fast-Fourier-transformed data and the latter half of the fast-Fourier-transformed data.
According to a ninth aspect of the present invention, a coherent integrating method of a radar system, comprises a step of extracting a receiving signal sampled within one pulse per transmission pulse from receiving signal data, a step of fast-Fourier-transforming the extracted receiving signals into two groups of data based on a former half of an observation time and a latter half of the observation time, a step of delaying the former half of the fast-Fourier-transformed data by a predetermined time in order to take synchronism of the former half of the fast-Fourier-transformed data and the latter half of the fast-Fourier-transformed data, a step of executing a complex conjugating operation with respect to one of two FFT processing results, a step of executing a complex multiplication of the other FFT processing result of the two FFT processing results and a result of the complex conjugating operation, and a step of repeatedly executing the processes up to the complex multiplication with respect to an arbitrary number of transmission pulses, and performing a complex addition of an obtained result of the complex multiplication for the every same Doppler frequency component.
According to a tenth aspect of the present invention, a coherent integrating method of a radar system, comprises a step of extracting a receiving signal sampled within one pulse per transmission pulse from receiving signal data, a step of compensating a phase so that an initial phase of the receiving signal coming from an observation target located at the same distance becomes zero, a step of fast-Fourier-transforming the receiving signal data after compensating the phase, and a step of repeatedly executing the processes up to the FFT processing with respect to an arbitrary number of transmission pulses, and performing a complex addition of an obtained result of the FFT processing for the every same Doppler frequency component.